The assessment of sperm motility is an indispensable means in the clinical diagnosis of male infertility. “Laboratory Manual For The Examination Of Human Semen And Sperm-Cervical Mucus Interaction” published by the World Health Organization (WHO) indicates several commonly used indicators of the clinical testing of sperm, including density test, morphological test and motility test. The criteria of sperm motility are as follows: Class a, progressive motility (≥25 μm/s straight line velocity at 37° C.); Class b, non-linear motility (5-25 μm/s straight line velocity at 37° C.); Class c, non-progressive (0-5 μm/s straight line velocity at 37° C.); Class d, immotile. The existing method of detection utilizes video recording by a microscope combined with Computer-aided Sperm Analysis (CASA) software to evaluate sperm motility. However, the existing microscopic observation method can only be used in the evaluation of sperm, but cannot screen and separate sperm in specific velocity range for subsequent in vitro fertilization (IVF) or research purposes.
The microfluidic chips have great potentials in the evaluation and screening of sperm. The flexible design of various micro-channel and micro-structure on a microfluidic chip makes it easy to mimic the physiological condition, manipulate or position the sperm, and control the fertilization process; in addition, because of the advantages of integration on a microfluidic chip, multi-parameter detection of sperm can be integrated on a single chip.
In 1993, Kricka el al. from University of Pennsylvania used a microfluidic chip to evaluate the motility of sperm for the first time (Kricka et al., Clin. Chem. (1993) 39: 1944-7). In 2004, researchers from University of Michigan filed a patent application (PCT Patent Publication No. WO 2004/108011 A1) to screen sperm with higher motility in a microfluidic chip, in which interfaces of fluid was achieved because of laminar flow in micro fluidics, so that sperm with higher motility can swim through the interface and reach the target region to be transported to the outlet pool. However, the methods disclosed in this patent application cannot screen sperm within a specific velocity range; besides, the chip has two inlets and two outlets and the fluid interface needs to be stable, thus the structure and the operation of the chip are both complicated. In 2004, researchers at National Taiwan University filed a patent application (U.S. Patent Publication No. 2011/0061472 A1) for the evaluation and screening of motile sperms, in which fluid flow was used against sperm swimming. Only sperms with high motility could reach the contracted region of the channel and be transported to the outlet. However, the fluid direction in the contracted region was the same as sperm swimming direction. The chip did not implement the capabilities of evaluating, screening and capturing sperm in different motility ranges. In addition, the contracted region was too narrow and only allowed one sperm to pass at a time, greatly reducing the throughput of sperm evaluation and screening.
In 1998, G. Fuhr el al. from Humboldt University, Germany reported a micro-electrode chip in Human Reproduction. (Fuhr et al., Hum. Reprod. (1998) 13: 136 41) They utilized negative dielectrophoresis to capture single sperm and they could capture sperm of different motility by applying different electrical field force. In 2010, A. Ohta et al. from University of Hawaii reported a micro-electrode chip with an integrated optical system in Lab on a Chip. (Ohta et al., Lab Chip. (2010) 10: 3213-7) The chip exerted force on sperm by optical tweezers in order to distinguish live and dead sperms. However, such methods using electric field force or light field force to screen and manipulate sperm may harm the sperm and make an impact of the health of the sperm during screening, which may result in the failure of subsequent in vitro fertilization or research work.
In recent years, a growing number of researchers found that clinical routine tests of sperm density, morphology and motility are not competent enough to reveal male reproductive capacity (e.g., Mona Bungum et al., Asian Journal of Andrology (2011) 13: 69 75). The latest research showed that parameters such as DNA integrity, acrosome ability, chemotaxis, etc. seriously affect the fertilizing capacity of sperm (Xie et al., Clin. Chem. (2010) 56: 1270-8). In other words, the fastest sperms may not be the sperm with the strongest fertilizing capacity. Hence the motility classification of sperm and the capture and screening of sperm in different velocity range are of great significance.